1. Field of the Invention
The present invention is broadly concerned with processes for oxidizing organic substrates (and oxidation reaction mixtures) wherein an oxidizable substrate/oxidation catalyst reaction mixture is volumetrically expanded through use of a compressed gas so as to facilitate and enhance oxidation of the substrate. More particularly, the invention in preferred forms pertains to oxidation reaction mixtures including an organic solvent system together with the substrate and oxidation catalyst, wherein an inert gas such as carbon dioxide is introduced into the reaction mixture for volumetric expansion thereof, followed by introduction of an oxidizing agent; volumetric expansion serves to accelerate the oxidation of the substrate.
2. Description of the Prior Art
During conventional homogeneous catalytic oxidation of organic substrates in organic solvents, the catalyst and substrate are dissolved in a suitable organic solvent medium such as methylene chloride or acetonitrile to form the initial reaction mixture. Dioxygen is then bubbled through the reaction mixture. The reaction proceeds in the xe2x80x9copenxe2x80x9d system until the desired conversion is achieved. However, these conventional processes suffer from several drawbacks. First, the upper temperature limit is constrained by the boiling point of the substrate and/or solvent medium because, at high temperatures, the resulting vapors can form explosive mixtures. Hence, the reaction must be carried out at lower temperatures whereat vapor pressures are low.
Another problem with prior art processes is that, at such low temperatures, reaction rates are low making large batch times or reactor volumes necessary during continuous processing. This large hold-up of hazardous materials in a reactive environment is unsafe.
The prior art processes are also lacking in that the solubility of dioxygen in the reaction mixture is so low that it is often the rate-determining step of the reaction. The lack of sufficient oxygen solubility can also adversely affect catalyst performance in selectively forming products. That is, the catalyst may be intrinsically superior for activating molecular oxygen, but the limited solubility of oxygen in the reaction mixture prevents the catalyst from exhibiting its full potential.
In recent years, supercritical CO2 has been employed as a replacement for organic solvents in homogeneous catalytic oxidation systems. Supercritical CO2 has also been exploited to overcome O2 solubility limitations. However, extremely high pressures are required to solubilize catalysts such as transition metal complexes in the supercritical CO2-based reaction mixture. Moreover, the conversions attained with supercritical CO2-based systems are usually lower than those obtained with conventional solvents. Thus, the only real advantage with prior art supercritical CO2-based systems is the replacement of conventional organic solvents with an environmentally-benign solvent.
The present invention overcomes the problems of the prior art and provides novel oxidation processes and reaction mixtures wherein the oxidation reaction mixtures broadly including an oxidizable substrate and an oxidation catalyst are supplemented with a compressed gas such as carbon dioxide so as to volumetrically expand the reaction mixture, thereby facilitating and accelerating oxidation. Although the expansion gas may also serve as the oxidizing agent, typically an oxidizing agent separate from the inert gas is employed. Also, the reaction mixtures generally include an organic solvent system.
In more detail, the inventive processes comprise forming a reaction mixture including an oxidizable substrate and an oxidation catalyst, volumetrically expanding the reaction mixture, and thereafter causing the oxidation reaction to occur. The volumetric expansion is normally carried out by introducing a compressed gas into the reaction mixture. If the inert gas is also an oxidizing agent for the substrate (e.g., when N2O is employed), the oxidation reaction proceeds. However, where an inert gas such as CO2 is employed, a separate oxidizing agent is introduced into the expanded reaction mixture to initiate the oxidation reaction.
In preferred forms, the starting reaction mixture includes an organic solvent system, with the substrate and catalyst dispersed (and preferably solubilized) in the solvent system. Such an organic system is usually made up of solvents selected from the group consisting of acetonitrile, methylene chloride, dimethyl sulfoxide, acetone, hexane, chloroform, toluene, dichloroethane and mixtures thereof. A solvent system made up of a mixture of acetonitrile and methylene chloride is particularly preferred.
A variety of oxidation catalysts can be used in the invention. For example, transition metal complexes of Fe, Mn, Co, Cu, Ni, V, Cr, Mo, W, Re, Ru, and Rh and mixtures thereof can be used to good effect. Especially preferred catalysts are those selected from the group consisting of Co(salen*), Co(salen), and PFTPPFeCl. Similarly, the class of useable substrates is very large, and may be selected from the group consisting of the phenols, alkenes, cycloalkenes, alkanes and alcohols, and mixtures thereof. The substrate may be present as a gas, a liquid, or as a solid.
The expanding gas is generally selected from the group consisting of carbon dioxide, N2O, Xenon, and SF6, although for reasons of cost and ease of use, pressurized subcritical or supercritical carbon dioxide is usually the gas of choice. The expanding gas is present in the reaction mixture at a level below that which will cause the catalyst to precipitate; that is, the catalyst is usually least soluble component of the reaction mixture, and for good results it should remain dispersed. Therefore, the expanding gas is introduced at levels which will maintain catalyst suspension. These level of course vary depending upon the components of the reaction mixture, and especially the catalyst. It is therefore usually necessary to preliminarily determine the extent of expanding gas supplementation which can be accommodated with each individual reaction mixture.
In most instances the expanded reaction mixture will have a lower density than the reaction mixture prior to introduction of the expanding gas. In such cases, the extent of lowering of the density is an alternate measure of the amount of expanding gas which can be used before the catalyst begins to precipitate.
A number of oxidizing agents can be used as required in the methods of the invention. The most common oxidation agents are selected from the group consisting of air, oxygen, ozone, N2O and mixtures thereof. Molecular oxygen is usually the preferred agent.
The processes of the invention are carried out in a closed system at superatmospheric pressures. These pressures range from about 20-250 bar, more preferably from about 50-200 bar. The oxidation reactions can be carried out over a wide temperature range, usually from about xe2x88x9270 to 250xc2x0 C., more preferably from about 15-100xc2x0 C.
The inventive processes are particularly useful for oxidizing phenols and alkenes using a transition metal complex as the oxidizing catalyst. Examples of such catalysts include [N,Nxe2x80x2-Bis(3,5-di-tert-butylsalicylidene)1,2-cyclohexanediaminato(2-)]cobalt(II) (hereinafter referred to as xe2x80x9cCo(salen*)xe2x80x9d) and [N,Nxe2x80x2-ethylenebis(salicylidene-aminato(2-)]cobalt(II) (hereinafter referred to as xe2x80x9cCo(salen)xe2x80x9d) and [5,10,15,20-tetrabis(pentafluorophenyl)porphyrin] iron(III) chloride (PFTPPFeCl).
The inventive processes have numerous significant advantages over prior art including: apply to a wide range of catalyst and substrates; require low quantities of organic solvents; increase oxygen solubility in the reaction mixture thus improving substrate conversion and selectivity; can be carried out at lower operating pressures than prior art supercritical CO2-based reaction systems; and are safer to carry out than prior art processes.